c/c++ questions

toovato · 02-26-2004, 11:07 PM

got two questions:

1) What does 3[x]++ do?

2) new to oo - have an assignment to write an object with the following conditions:
The door can have two states (open or closed)
- Create methods for finding the state of the door (open or closed)
- Create methods for opening and closing the door
State any assumptions and additional features you have made on the Door class.

Thanks in advance,

ElementNine · 02-26-2004, 11:25 PM

Hmm i think that would cause an error. var[x]++ would return a value one higher that the one pointed to by x so if var[x] = 3 then var[x]++ would return 4

you wanna create a door class and probably have a variable "doorstatus" and two methods "void setDoorState(boolean)" "bool getDoorState(void)" setDoorState take a boolean parameter and sets the variable "doorstatus" to true or false getDoorState would just return doorstatus

toovato · 02-26-2004, 11:29 PM

thanks for you help - could you provide a code snippet?

eshwar_ind · 02-26-2004, 11:34 PM

3[x]++;
just increments the value in x[3];

ElementNine · 02-26-2004, 11:44 PM

Code:

class Door {
       public:
               Door(); //constructor
               void setDoorStatus(bool);
               bool getDoorStatus();
        private:
               bool doorStatus;
};

Door::Door(){
        doorStatus = true; //true for open false for closed
}

void Door::setDoorStatus(bool ds){
         doorStatus = ds;
}

bool Door::getDoorStatus(){
         return doorStatus;
}

I think that should work, someone let me know if i messed up but im pretty sure thats right

edit sorry that smiley is actually a colon and a D but it turned into a smiley

ElementNine · 02-26-2004, 11:48 PM

eshwar are you sure that doesnt cause an error it looks like he's trying to increment a value inside an array named 3 which is an error but if he meant x[3]++ then you and i are right it would add one to the value of x arrays 4th value

toovato · 02-26-2004, 11:59 PM

you have been a great help - thanks a million

eshwar_ind · 02-27-2004, 12:09 AM

Mr. Element Nine
both are same.
3[x] is same as x[3].
When it comes to 3[x]++, 3[x] is evaluated first because [] has highest precedence than ++.
So there is no problem .
3[x]++ is same as x[3]++.
what do u say. check it.

ElementNine · 02-27-2004, 12:34 AM

uh unless everything ive learned has been wrong, that looks like an array correct? arrays structure is var[index] and variables cant start with a number

eshwar_ind · 02-27-2004, 12:47 AM

yah what ever you said is correct. you might be thinking that X[3] as a variable name, No its not. i think you know very well variable names cant have a charcter '[' like this. valid names contain only alphabets, _ , 0....9.
and one more rule is your variable name shouldnt be started with a number.
actually X[3] is not a variable name. X is the name of array. Its the base address of array( we cant change X value). no other names are there when we declare an array as below
int X[3];

ex:
datatype X[10];
when you use X[3], it is expanded as *((datatype *)X+3)
did u get my point?
ask me if u have any doubts regarding this.
bye,
Eshwar.

eshwar_ind · 02-27-2004, 12:56 AM

Hi thats an equivalent expression. I am not sure whether that is expanded like that or not by compiler.
but its equivalent is absolutely correct.

toovato · 02-27-2004, 01:20 AM

so which is right?

eshwar_ind · 02-27-2004, 01:28 AM

Both are correct.

cjcuk · 02-27-2004, 02:24 AM

Quote:

Originally posted by ElementNine
uh unless everything ive learned has been wrong, that looks like an array correct? arrays structure is var[index] and variables cant start with a number

They are expanded to *(argv + 1) ( for example ). The thing you do not seem to realise from that is that `argv' is in fact a number, as are all other variables. They are memory references.

vasudevadas · 02-27-2004, 09:16 AM

Quote:

Originally posted by ElementNine
var[x]++ would return a value one higher that the one pointed to by x so if var[x] = 3 then var[x]++ would return 4

Actually, if var[x] == 3 then var[x]++ will return 3. It will increment it after the operation. If var[x] == 3 then ++var[x] will return 4.